1. Field of the Invention
The present invention relates generally to a video disk player and a method for reproducing a video disk and, more particularly, is directed to a so-called slide servo or sled servo control apparatus for use with a video disk player.
2. Description of the Prior Art
In an optical type video disk player, when a photo pickup head is moved in the radius direction of the video disk at a speed higher than that in the normal playback mode, e.g. during a track scanning operation, while the photo pickup head is controlled by the tracking servo, an objective lens provided within the photo pickup head tries to remain directed toward the original track against the movement of the photo pickup head. Accordingly, the position of the optical axis of the objective lens (the position in the track width direction seen from the photo pickup head) is changed as shown by a corresponding waveform of FIG. 1A.
FIG. 1A shows the waveform of the tracking error voltage supplied to a drive coil (tracking coil) of the objective lens. In FIG. 1A, the abscissa indicates a moved amount (passed time) of the photo pickup head, whereas the ordinate indicates the position of the optical axis of the objective lens (tracking error compensating voltage Vc). As shown in FIG. 1A, during a period Td, tracking is correctly carried out and at this time the position of the optical axis of the objective lens is gradually deviated from the optical path of a laser light source in accordance with the movement of the photo pickup head. Soon after, the tracking servo can not follow such displacement, or the deviated position of the objective lens relative to the optical path of the laser light source reaches a limit in which the signal can be reproduced so that, during a period Tj, a tracking error compensating voltage Vc is forced to change to effect a so-called track jump. In the following description, a track jump means that the center of the objective lens is caused to jump in the width direction of the track by utilizing a moving coil.
In that case, owing to the track jump, the optical axis position of the objective lens is returned to the center of the visual field. In FIG. 1A, a value S corresponds to the visual field of the objective lens and is equivalent to about.+-.250 tracks.
Generally, the aforementioned tracking servo is referred to as a slide servo or sled servo, and the outline thereof will now be described with reference to FIG. 2. A spindle motor 1 rotates an optical disk 2, and a laser beam emitted from a laser light source 3 is inputted to an optical block 4.
The introduced beam travels through a half mirror 5 or the like and is emitted from an objective lens 6 as a beam B to irradiate tracks T on the optical disk 2. Two such optical blocks 4 are shown in the figure for purposes of illustrating the operation of the device, but it is to be understood that only one such block exists in actual practice The block 4 on the left side of FIG. 2 represents the situation where the beam B is centered over a given one of the tracks T whereas the rightmost block 4 represents the situation where the beam B initially is centered between two tracks T.
In the latter case, a DC component in a tracking signal Vt, obtained from a reflected beam (not shown) on the optical disk 2 is detected by a detector 7. Then, an error compensating signal Vc corresponding to the error amount therefrom is supplied to a tracking coil 8 as a tracking control signal such that the central position of the objective lens 6 always coincides with the center of the tracks T. A correctly centered beam B' is emitted from the objective lens, now denoted as 6', that is deviated to one direction from the position of the objective lens 6 forming an optical path of the beam B at the central position. This objective lens 6 is located at the limit position of the movable range. In FIG. 2, although the diameter of the light bundle of the beam is about 6.6 mm relative to the diameter (4.5 mm) of the objective lens 6 in practice, these relations are simplified for convenience.
In FIG. 2, an arrow S represents the direction in which the optical block 4 is moved by a sled motor (not shown)
In the period Td (FIG. 1A), the tracking of the objective lens is correctly carried out, so that the picture is displayed by a reproduced signal. In the period Tj, the tracking of the objective lens is not correctly carried out so that the spindle servo control of disk rotation can not correctly carried out. Consequently, in the period Tj, the reproduced signal is muted to provide, for example, a gray screen.
In the scan mode, therefore, the ordinary reproduced picture in the period Td and the muted picture in the period Tj are alternately displayed, or the ordinary reproduced picture in the period Td is intermittently displayed in response to the moving speed of the photo pickup head, whereby a reproduced picture in the fast forward or fast rewind mode can be obtained. This operation mode will be referred to as "scan mode" or "scan reproducing mode".
A constant angular velocity (CAV) system and a constant linear velocity (CLV) system are two known types of recording systems for a video disk. In the CAV system, the revolution rate of a disk is constant, for example, 1800 r.p.m., and per revolution, a video signal of one frame is recorded and a synchronizing signal is recorded in the radius direction. The CAV system provides a special playback function such as a still picture mode and the like, and the playback time is a maximum of 30 minutes for one side of a 30-cm disk.
In the CLV system, a video signal is recorded at a constant linear velocity of about 11 m/sec. The revolution rate at which a video signal is read-out from the inner periphery of the disk is 1800 r.p.m. (one frame per revolution) whereas the revolution rate in which a video signal is read-out from the outer periphery of the disk is 600 r.p.m. (three frames per revolution). Further, the revolution rate when a video signal is read-out from the intermediate portion of the disk is 900 r.p.m. (two frames per revolution) and the maximum playback time is 60 minutes for one side of the 30-cm disk.
FIG. 3 shows the location of a video signal recorded according to the CLV system. It is to be understood from FIG. 3 that two video fields are recorded in the innermost portion of the disk and that 6 fields are recorded in the outermost portion of the video disk.
In the case of the video disk recorded according to the CLV format, each time the video disk is rotated, an angular position in which a vertical synchronizing signal is recorded (shown by a solid circle in FIG. 4) is deviated little by little so that, when the video disk recorded according to the CLV format is reproduced in the scan mode, a picture is reproduced with a disturbed vertical synchronization. A track interval is illustrated in an enlarged-scale in FIG. 4, and therefore, the position of the vertical synchronizing signal is considerably changed.
Assuming that P is the track pitch=1.67.times.10.sup.-6 (m), R is the radius of the innermost periphery of the track=55.times.10.sup.-3 (m) and that N is the track number=1 to 54000, then the length L of N'th track is determined as EQU L=2.pi.(R+P (N-1)) (m) (i)
Video signals of two fields are recorded in the innermost peripheral track of the video disk according to the CLV format and one length of track is fixed as .pi.R so that the number F of the fields involved in the N'th track is expressed by the following equation: ##EQU1## Since k.ident.2P/R.apprxeq.60.73 (p.p.m), N=1 yields F=2 fields, and N=54000 yields F.apprxeq.5.28 fields
The value k in the equation (ii) represents the amount by which the angular position in which the vertical synchronizing signal is recorded is changed when one track jump is carried out. That is, the value k indicates an amount in which the phase of the reproduced vertical synchronizing signal is changed.
The relationship between the track and the vertical synchronizing pulse Vsync in FIG. 4 will be described more fully with reference to FIGS. 5A to 5C.
FIG. 5A shows a condition near a track portion where two video fields are recorded in one track, wherein vertical synchronizing pulses Vsync are aligned substantially in the radius direction of the disk with a reference vertical synchronizing pulse (shown by a chain line) which is generated by the video disk player. FIG. 5B shows a condition near a track portion in which three video fields are recorded in one track, wherein although the vertical synchronizing pulses Vsync are aligned substantially in the radius direction of the disk similarly to FIG. 5A, the phase of the reproduced vertical synchronizing pulses Vsync is deviated with a deviation of several 10s of percents from the phase of the reference vertical synchronizing pulse. This is because a spindle servo is applied so that the phase of the vertical synchronizing pulse of the reproduced video signal coincides with the reference phase. In that case, the phase of the vertical synchronizing pulse in the recorded signal is considerably deviated from the phase of the reference vertical synchronizing pulse so that, in the scan mode, the photo pickup head can not be pulled into a servo controllable range of .+-.3% by the servo control. If the photo pickup head is jumped into a track in this area, then a picture can not be immediately reproduced and so, this area is referred to as a dead zone track area.
FIG. 5C illustrates a condition near a track portion where 2.7 video fields are recorded in one track. As shown in FIG. 5C, the recorded positions of the vertical synchronizing pulses are scattered around the recorded positions of the reference vertical synchronizing pulse. If the track shifting is carried out for at least 10 or more tracks near this track portion, then at least a vertical synchronizing pulse of a certain track falls within the pull-in range of the servo control.
It is to be understood from FIGS. 5A to 5C that, when a track jump is carried out near the tracks of F =2, 3, 4 and 5, if the number of tracks jumped is small (9 tracks in FIGS. 5A to 5C), the phase change of the vertical synchronizing pulse in the reproduced video signal after the track jump is small as compared with that occurring before the track jump. It is needless to say that, even when a track jump is carried out near the tracks of F=2, 3, 4 and 5, if the number of jumped tracks is selected to be very large, then the phase change of the vertical synchronizing pulse after the track jump is carried out can be made large. In this connection, the number of tracks in which 2, 3, 4 and 5 fields of the video signal are recorded in one track is expressed by the following equation ##EQU2## If F=2, 3, 4 and 5 is substituted into the above equation (iii), this yields N=1, N=16468, N=32935 and N=49402, respectively.
When a track jump is carried out near the track of F=2.5, 3.5, 4.5, . . . , even if the number of tracks jumped is small, the phase of the reproduced vertical synchronizing pulse after the track jump is considerably changed as compared with that changed before the track jump. In the case of FIG. 5C, the phase of the vertical synchronizing pulse is considerably changed only by a track jump of one track from N=11500, whereby the vertical synchronizing pulse enters a region of .+-.3%.
In the following description, an area in which the phase change of the vertical synchronizing pulse after the track jump is small and in which the phase of the reproduced vertical synchronizing pulse after a track jump of 10 or more tracks can not reach the phase of the reference vertical synchronizing pulse is referred to as "dead zone track area". Accordingly, if the dead zone track area of the video disk recorded according to the CLV format is reproduced in the conventional scan reproduction mode, a picture is reproduced wherein the vertical synchronization thereof is disturbed.
To avoid this defect, it is proposed that, in the scan reproduction mode, if the phase of the vertical synchronizing pulse contained in the reproduced video signal is continuously large (more than .+-.3%) relative to the phase of the reference vertical synchronizing pulse, the above vertical synchronizing pulse is removed and a new vertical synchronizing pulse is inserted into a position in which a time series is continued. In this proposal, however, although the vertical synchronization is not disturbed, a black band corresponding to the removed vertical synchronizing pulse appears in the reproduced picture or the upper half portion and the lower half portion of the reproduced picture are reproduced in the picture screen in a vertically reversed fashion. These conditions are represented in FIG. 6 and FIGS. 7A to 7C wherein FIG. 6 illustrates the vertical synchronizing pulse PBV and the reference vertical synchronizing pulse REFV of the reproduced picture, whereas FIG. 7A to 7C illustrate the examples of the reproduced pictures monitored on a television receiver.